Virtually from the beginning of recorded history man has required the means to determine true horizontal and vertical in major construction of all types. This need has continued undiminished to the present time, and in fact the primitive carpenter's level has persevered virtually unchanged for many years. In such a level a vial or generally cylindrical container is partially filled with a liquid so a bubble of air remains. When the vial is place horizontally on a level surface, i.e., when the axis of the vial is coparallel with the local horizon, the bubble will be equidistant from the ends of the cylinder, i.e., it will be centered. Any deviation from level will be manifested by deviations of the bubble from centrality, i.e., it will be closer to one end than the other. In this application, horizontal level will mean a line coparallel to the local horizon, and vertical level will mean a line perpendicular to the local horizon.
The classical bubble level has limitations in accuracy as well as convenience. Because it relies on visual sighting when used in, e.g., carpentry the user's eyes may need to continually shift between the level and the focus of his work, such as the end of a beam. Consequently, there is a need for a level which can be used by one person without attendant constant close eye contact. It is desirable that such a level be responsive, both in the sense of a rapid response time to changes in horizontal position, and in the sense of having the capability of detecting small deviations from absolute level. Additionally it is paramount that such a level be sturdy, inexpensive, simple to manufacture, and convenient to use under extremes of field conditions such as temperature, humidity, and cramped work space.
Although there are examples of levels and level sensing units based on electroptical devices capable of great precision and accuracy such articles are excluded from further consideration here because of their cost and sometimes because of their inconvenience in operation. The construction worker, for example, needs a much simpler device. Perhaps the best example of an attempt to fill this need is that described in U.S. Pat. No. 4,152,839, which utilizes a sealed capsule containing a bead of mercury which acts much like an air bubble in a vial of liquid. When in a horizontal level position mercury, an electrical conductor, is at the center and in electrical contact with electrodes which energize a current indicating a level status, e.g., a buzzer or bulb. When in a position deviating from horizontal the bead of mercury tends to roll away from the center, breaking electrical contact and deenergizing the circuit. The many possible variations of this theme need not be elaborated on at this time.
The above design is essentially a mercury-actuated switch. An acknowledged limitation of such a switch is its relatively sluggish response and insensitivity to small changes, both arising because the extremely high surface tension of mercury requires a relatively large degree of angular change from level for movement of the mercury bead. Although somewhat of an exaggeration, it is almost as if such a mercury-actuated switch exhibits a step response rather than a continual one to changes in angular position of the mercury-containing capsule.
In U.S. Pat. No. 4,685,218 I have devised a simple but accurate level sensing unit with a rapid response time and which can readily drive or be incorporated into an electrical circuit indicating the state of level. The level sensing unit is easily incorporated into, for example, a contractor's level to afford a device which is significantly advantageous relative to prior art levels. As significant as is the advance presented by this level sensing unit, nonetheless certain improvements were judged to be desirable. One goal was to reduce the sensitivity of operation of the level to expansion and contraction of the liquid exerting a buoyant force on floats connected to electrical contacts. A second goal was to make the operation virtually independent of the specific gravity of the liquid providing the buoyant force, and preferably to make a level sensing unit operate effectively with a buoyant liquid of specific gravity of about 1 or even less. Although the prior art level sensing unit worked even with a liquid of specific gravity near 1, it operated best when the specific gravity was at least 1.5. Another goal was to further improve the response of the unit to changes in liquid orientation. Thus, perturbations in liquid orientation imparted some tendency to the float of the prior liquid sensing unit to oscillate like a pendulum.
Upon considering this wish list of improvements it occurred to me that all could be achieved if the unit bearing the moving contacts was on an axis so that the resulting torque about the axis was zero regardless of the unit's orientation. Stated differently, the requirement is that the unit's pivot points be coincident with an axis with respect to which the unit has a zero moment of inertia. Such a requirement is equivalent to every plane intersecting and normal to the axis coincident with the pivot points (or axle) being symmetric with respect to inversion about the point of intersection. With such an arrangement motion about the axle is unattended by oscillation. If the unit has the same symmetry along a second axis normal to the first axis mentioned above then it also will be relatively insensitive to the contraction and expansion of the liquid exerting the buoyant force and to the specific gravity of that liquid. My invention is just such a liquid sensing unit.
For the purpose of this application, inversion symmetry means that every plane of a body normal to an axis of rotation is invariant to inversion about a point in the plane representing the intersection with the rotational axis. Stated differently, such planes of a body are invariant to the transformation p(r,.theta.).fwdarw. p(r,.theta.+180.degree.) where r and .theta. are the polar coordinates of point p.